1. Field of the Invention
This invention relates to an optical element and a scanning optical apparatus using the same, and in particular to an optical element whose refractive index is distributed internally. Such an optical element is particularly well-suited for scanning optical apparatuses according to the invention, such as a laser beam printer having an electrophotographic process, or a digital copying machine, in which a light beam modulated and emitted from light source means is reflected by a deflector comprising a polygon mirror revolving, and the deflected beam scans a surface for recording via an image formation optical system as the f.theta. lens having f.theta. characteristics, whereby image information is recorded.
2. Description of the Related Art
Conventionally, in a scanning optical apparatus such as a laser beam printer, a light beam modulated in accordance with an image signal and emitted from light source means is periodically deflected by a deflector comprising a rotatable polygon mirror, and the deflected beam is converged into a beam spot on the surface of a photosensitive recording medium such as a photosensitive drum by an image formation optical system having f.theta. characteristics, so that the beam spot scans the surface to record an image.
FIG. 1 is a schematic perspective view illustrating the main components of a conventional scanning optical apparatus.
In FIG. 1, a divergent light beam emitted from light source means 11 is formed into a substantially parallel beam by a collimator lens 12 to be controlled as to its light amount by a diaphragm 13 to be incident on a cylindrical lens 14 having a predetermined refractive power only in a subscan direction. The parallel beam incident on the cylindrical lens 14 passes through the lens as the parallel beam in a main scan section, while the parallel beam incident on the lens converges in a subscan section to form a substantially linear image on a deflection surface 15a as a reflection plane of a deflector 15 comprising a rotatable polygon mirror.
The beam deflected on the deflection surface 15a of the deflector 15 is guided onto a photosensitive drum surface 18 as a scanned surface via an image formation system 16 which has f.theta. characteristics, and the deflector 15 is revolved in the direction of the arrow A, so that the guided beam optically scans the photosensitive drum surface 18 to record image information.
FIG. 2 is a section view illustrating the main components in the subscan direction, which is perpendicular to the main scan direction including the optical axis of the f.theta. lens, shown only between the deflector and the scanned surface of the conventional scanning optical apparatus.
In FIG. 2, the deflection surface of the deflector 15 and the photosensitive drum surface 18 as the scanned surface are substantially optically conjugate to each other, as to the f.theta. lens 16. Therefore, if there is any so-called surface inclination in the deflector, such inclination is corrected by forming the beam into an image in the same scan line on the photosensitive drum surface.
Conventionally, in designing a scanning optical apparatus, as shown in, for example, in Japanese Patent Application Laid-open No. 61-190312, there is proposed a scanning optical apparatus designed such that a Gaussian imaging plane 82 as a paraxial image plane is positioned in the back of the scanned surface (i.e., opposite to the deflector) in consideration of spherical aberration of the optical system. Such an arrangement positions a so-called "best image plane" 81 directly on the scanned surface 18. In other words, the optical apparatus is designed such that the focal length calculated based on the arrangement of the optical system, the curvature of its plane, its refractive index, and its thickness, is longer than the focal length actually measured.
In FIG. 2, solid lines represent the beam travelling to the best image plane 81, and a broken line represents the beam travelling to the Gaussian imaging plane 82. In this specification, the Gaussian imaging plane is the image plane calculated based on the position of an object, the position of the light source means, the curvature of the lens, the refractive index of the lens, the thickness of the lens, the position of the lens surface, and so forth.
Most recently, f.theta. lenses have been produced by plastic molding. Apparatuses using such lenses must therefore consider not only optical aberrations but also internal distortions caused by the plastic molding process.
In order to mold the lens without internal distortions, it is considered to cool the material of the lens, very slowly. This method however has a problem that lead time lengthens, and productivity deteriorates.